Tokyo readies for the rise of the 'guerrilla rainstorm'

Tokyo readies for the rise of the 'guerrilla rainstorm'

Japan’s capital may be known for its earthquake resilience strategies, but a recent increase in intense, sudden storms poses another threat – one that the city’s flood protection systems may not be able to manage

When people speak of natural disasters in Tokyo, they usually mean the major earthquake that seismologists say will eventually strike the Japanese capital. But for the local government, weather-related disasters are as much if not more of an immediate worry. The city’s coastal location puts it in the path of the Asia-Pacific region’s most violent typhoons – and there has been a measurable increase in the strength of rainstorms in recent years.

Tokyo’s rainfall in 2014 was about 20% above normal, but the kind of storms that have struck the capital lately are different than they were in the past. Many are caused by huge cumulus clouds that form quickly and in succession when moist air from the ocean comes up against the warm air trapped among tall, closely packed buildings. Locally, these sudden, intense downpours are called “guerrilla” storms, because they seem to attack out of nowhere.

Two years ago, four workers who were reinforcing storm sewers drowned when such a storm hit unexpectedly. The main problem in these situations is that Tokyo is covered with concrete and has many levels of underground infrastructure: there is no ground soil to absorb water. The city has two sewer systems, one for runoff and one for sewage, and when rainfall exceeds 50mm/hour the water is diverted automatically into the sewage channels, which fill up and have to be diverted into the sea, meaning raw sewage ends up in the environment without being treated. Last year, during one storm, Taito Ward in eastern Tokyo received 150mm in one hour.

Tokyo is covered with concrete … there is no ground soil to absorb water

About 30% of the prefecture’s population lives below sea level, mostly along Tokyo Bay or the many rivers that feed into it. The levees that were built roughly 300 years ago to protect the downstream areas haven’t always been sufficient. In 1948, Typhoon Kathleen submerged most of the northwestern portion of the city under three metres of water when the Tone River breached its banks. More than 1,000 people died and 31,000 homes were destroyed. The storm was considered a once-in-a-lifetime phenomenon, and the low-lying areas have become even more populated in the intervening years, with landfill extending far into the bay. Levees have been reinforced, but the metropolitan government knows it is not enough.

One of the biggest recent flood prevention projects is an underground discharge channel that was completed six years ago about 25 miles north of the city in Kasukabe. The structure feeds overflow from five rivers into a man-made underground reservoir through a huge shaft and a tunnel more than 4 miles in length. Four pumps can handle as much as 200 cubic metres of water a second. The mechanism goes a long way toward diverting excess water that flows down from the upper reaches of the Kanto Plain, but may have less effect in the event of heavy rains that fall directly on the city.

The pressure-controlled water tank at the Metropolitan Area Outer Underground Discharge Channel in Kasukabe, north of Tokyo. Photograph: Ho New/Reuters

Ward governments have asked private property owners to build special runoff systems under their land so that when there is excess rain it can be absorbed into the soil, alleviating the city’s burden for runoff. Some even offer to subsidise such construction, but without regulations that compel landowners to do so, few have undertaken the work.

Experts don’t think that structural precautions alone can safeguard a city such as Tokyo from extraordinary storms

Another flood-prevention project is the “super levees” that the central government is attempting to build along 540 miles of six rivers. The work involves increasing the elevation of existing levees. But if they are made higher, the base has to be widened in order for them to maintain their structural integrity, and since tightly packed residential neighbourhoods are already situated along the levees, houses have to be condemned and residents forced to move when the work is carried out. Japan has a very weak concept of eminent domain, and between the late 1980s when the project began and 2012 when it was suspended temporarily due to budget concerns, only 5.4% of the planned work had been done. At that rate it will take 400 years to complete, and many people in and out of the government believe the project is merely a public works boondoggle that has more to do with enriching construction companies than with saving lives.

In any case, experts don’t think that structural precautions alone can safeguard a city such as Tokyo from extraordinary storms. This spring, Japan’s public broadcaster, NHK, aired a series of special programmes on “mega-disasters”, one of which addressed guerrilla rainstorms and “super typhoons”. Worst case scenarios were simulated using computer graphics, showing how subways and underground spaces would be completely inundated and streets submerged under at least three metres of water. Though such a situation would be rare, it is also seen as inevitable. Consequently, more research has gone into predicting heavy rainfall, especially when it’s focused on a small area.

As NHK pointed out, the best way to save lives is to make sure residents know when a storm is coming and what they can expect in their area. Local governments distribute “hazard maps” that tell residents whether or not their localities are subject to inundation, whether by overwhelmed sewers or by overflowing tributaries. Combined with a storm prediction system, residents can evacuate when a warning is given. This summer, a system devised by the Ministry of Land, Infrastructure, Transport and Tourism that predicts rain storms of more than 30mm/hour is being tested that can give ten minute warnings to affected areas.

Ten minutes isn’t much time, but if people decide to evacuate, where do they go, especially if they live in a neighbourhood that’s below sea level? Local governments designate evacuation points in multi-story public buildings, such as schools, but depending on where a resident lives it could be a long walk – and authorities warn against moving around outside during a flood. Some communities are making their own arrangements. Yoshii Ishii, the 83-year-old head of the neighbourhood association of Minami Senju, a cramped warren of wooden houses located near the Sumida River, told us three years ago that he had come to an understanding with several high-rise condominiums that had been built in the area since 2000. “We had a meeting,” he said. “They said we can use their building in the event of an emergency.”

The proliferation of tower condos in recent years has been controversial for their performance in earthquakes, but tall buildings also exacerbate the heat island phenomenon that contributes to guerrilla storms. For the residents of Minami Senju, however, skyscrapers make up for the flooding they could be indirectly causing by providing higher, drier ground.